The present invention relates to imaging by the nuclear magnetic resonance method (NMR) which is at present undergoing rapid development, particularly for the examination of the human body. It makes it possible to obtain images or pictures of the body which have a quality and accuracy which have not been obtained hitherto with conventional X-ray radiology methods. It applies to the phase of building up the image obtained by spatial coding and calculation with the aid of a computer.
The NMR imaging method uses the nuclear magnetic resonance property of certain nuclei present in the human body, essentially the protons distributed in any organism, and certain nuclei which are much less abundant but of biological interest, such as phosphorus .sup.31 p, potassium .sup.39 K and sodium .sup.23 Na. By forming a chart or map of their concentration at each point of the volume examined, it is possible to produce images of living tissues, particularly from the hydrogen contained in water and the lipids forming the two essential components of any living matter.
A medical NMR installation is essentially comprised of a magnet for producing a static, uniform magnetic polarization field Bo throughout the zone of the body to be examined and on this is superimposed with the aid of an auxiliary coil, radiofrequency field pulses in a plane perpendicular to the direction of field Bo.
The images are obtained by usually resonating the hydrogen nuclei or protons contained in the biological tissues. This resonance is possible because each proton behaves like a microscopic magnet and the radifrequency field is equivalent to two fields rotating in reverse directions and whereof that turning in the precession direction of the spins is capable of coupling therewith. When the static field Bo is applied, the spins are all oriented parallel to the axis of the field. Thus, there are no more than two possible orientations, namely in the direction of the field or in the opposite direction. Moreover nothing happens on applying the radiofrequency field at a random frequency. However, if said field is chosen equal to or very close to f.sub.o (Larmor frequency), such that 2.pi.f.sub.o =.gamma.B.sub.o, in which .gamma. is a physical constant characterizing the nuclei which it is wished to resonate and called the gyromagnetic ratio, the coupling of the spins and the rotary field reaches a value such that the latter resonate.
The resonance signal detected during their return to equilibrium in free precession is proportional to the magnetization M of the nuclei placed in the magnetic polarization field Bo. Observation of the resonance phenomenon requires the presence of a high magnetic field (0.1 to 1 Tesla), which is very uniform throughout the volume to be imaged.
Reference will now be made to a certain number of known phenomena and reference will be made to a certain number of conventions facilitating the understanding of the remainder of the text. Firstly the volume to be imaged will be referenced with respect to a trirectangular trihedron O,x,y,z in which the axis Oz is, by convention, parallel to the d.c. polarization field Bo. In this hypothesis, the macroscopic magnetization vector M of the medium to be imaged is, in the inoperative state, parallel to axis Oz and the radiofrequency field B.sub.1 rotating at the angular velocity .omega..sub.o will be in plane x,o,y, which is also that in which the resonance signal is collected.
In order to describe these various phenomena, it is often necessary to use a reference system x',O,y', x turning around Oz at angular frequency or ripple .omega..sub.o (FIGS. 1 and 2) and in this reference system B.sub.1 is aligned with Ox'.
A radiofrequency pulse at the Larmor frequency f.sub.o directed along axis Ox' has the property of rotating the macroscopic magnetization vector M by an angle .theta. about Ox in plane y'Oz, such that .theta.=.gamma.B.sub.1 t, in which .gamma. is the gyromagnetic ratio, B.sub.1 the modulus of the radiofrequency field and t the time during which said field is applied.
In NMR methods, particular use is made of pulses in which .theta.=90.degree. and .theta.=180.degree., called respectively the 90.degree. pulse and the 180.degree. pulse. These two pulse types respectively have the following properties.
The 90.degree. pulses along Ox' will tilt vector M along axis Oy', as can be seen in FIG. 1. On the basis of this initial position, vector M precesses in plane xOY, also with ripple .omega..sub.o =2.pi.fo like B.sub.1 and this is the movement which induces the resonance signal at the Larmor frequency and this is detected with the aid of a not shown coil placed in said plane xOy.
At the instant of applying a 90.degree. pulse along Ox', all the magnetic moments of the different nuclei are brought into phase along Oy' and vector M has its maximum modulus M along Oy'. As time passes, there is a dispersion of the magnetic moments in accordance e.g. with two components M.sub.1 and M.sub.2 rotating in opposite directions in plane x'Oy' relative to Oz taking place under the double influence of the inhomogeneity or heterogeneity of the field B=B.sub.o +E (x,y,z) and interactions between the nuclei. It is the transverse relaxation phenomenon which leads to the resulting moment M decreasing with a time constant T.sub.2 so that .vertline.M.vertline.=M.multidot.e.sup.-t/T.sbsp.2 and the resonance signal is cut out.
The 180.degree. pulses have the double property of realizing a population inversin of the spins by placing the macroscopic magnetization vector Mo in its antiparallel position and performing a symmetrical transformation with respect to the direction of said pulse, in the manner of the image in a plane mirror, of the magnetization vectors existing at the time of the pulse. Thus, in the case where the 180.degree. radiofrequency pulse is directed along Oy', this changes to -.theta. the phases .phi. of the vectors M.sub.1 and M.sub.2 which are then mutually interchanged and changes .phi. into (.pi.-.phi.) in the case where said 180.degree. pulse is directed like B.sub.1 along Ox', and M.sub.1 M.sub.2 then taking the places of M.sub.3 and M.sub.4.
A description will now be given of the known methods for obtaining a NMR image of a living organism by calculation. If the d.c. polarization field Bo is perfectly homogeneous, all the magnetization vectors M from the different points of the volume to be imaged are in phase and it is impossible in the total signal received: ##EQU1## to attribute to each point a particular frequency and intensity. In order to be able to identify the contribution to this total signal of each elementary volume, there is a frequency coding of the space to be imaged by superimposing on field Bo additional fields varying linearly along one of the coordinates, so that the component Bz has constant spatial gradients (.differential.Bz/.differential.x), (.differential.Bz/.differential.y) and (.differential.Bz/.differential.z) along x, y and z satisfying the three following equations: ##EQU2## in which Bo+Bz is the component of the d.c. polarization field along Oz and Gx, Gy, Gz are the three constant gradients of the resulting induction along Ox, Oy and Oz. In NMR imaging it is standard, but not completely correct usage to call these additional fields "linear gradients" and this term will be used throughout the remainder of the present text.
The sought objective of these linear gradients is readily apparent. The Larmor frequency or ripple .omega.=.epsilon.B is at all points of the volume to be imaged proportional to the induction at this point, so that spatial coding of this volume in phase and in frequency takes place and it is possible to show that the Fourier transform of the time signal s(t) or frequency spectrum s(.omega.) of said same signal is the three-dimensional image of the magnetization density sample, because in the presence of a linear gradient the frequency represents the coordinate along which said gradient is applied. For illustration purposes, FIG. 3 shows the distribution of vector B.sub.z in the case of a linear gradient G.sub.x in direction Ox. This is the method for building up the image called the direct Fourier method. It is applicable to three-dimensional imaging or 3D,FT and two-dimensional imaging or 2D,FT and a brief description will now be given of the principle, because it is to this imaging method that the present invention specifically applies.